The present invention relates to a reactor power control apparatus of a natural circulation boiling water reactor and a feed water control apparatus and nuclear power generation plant, and more particularly, the reactor power control apparatus and the feed water control apparatus for automatic control of the reactor power in a natural circulation boiling water reactor in which coolant is circulated by natural circulation, as well as to a nuclear power generation plant which is controlled by the feed water control apparatus
Generally, boiling water reactors are largely divided into a forced circulation type and a natural circulation type based on the circulation method for the coolant (cooling water). A forced circulation boiling water reactor (referred to as forced circulation reactor hereinafter) includes a jet pump or an internal pump or the like. This pump forces cooling water into the core placed in a reactor pressure vessel.
Meanwhile, a natural circulation boiling water reactor (called natural circulation reactor hereinafter) does not include a pump which circulates the cooling water by force as in the case of the forced circulation reactor. In the natural circulation reactor, the cooling water is circulated by the natural circulation force which is based on the difference in density (head difference) of the cooling water outside of a core shroud which surrounds the core and the two-phase flow including the steam and the cooling water inside the core shroud.
In the prior art, reactivity adjustment of the boiling water reactor (BWR) is performed by inserting a control rod containing a neutron absorber into the core or withdrawing the control rod from the core, and adjusting recirculation flow rate (core flow rate) of the cooling water that is also a neutron moderator, through the core. During the power increase step in start-up time, the period between the beginning of start-up and generator synchronization, the reactivity adjustment is performed by the withdrawing operation of the control rod. After generator synchronization, the reactivity adjustment is performed by the withdrawing operation of the control rod and by increasing the recirculation flow rate. In start-up time, in addition to this reactivity adjustment, because the auxiliary devices must be started up and the valves must be opened and closed, the burden on the operator is great. As a result, in an advanced boiling water reactor (ABWR), control rod operation and recirculation flow rate operation are automated, and thus the burden on the operator is reduced.
For example, in Japanese Patent Laid-open No. Hei 6-2902217, the reactor power is controlled by control rod being operated by the control rod drive apparatus, and the cooling water recirculation flow rate being controlled by the internal pump. The control rod drive apparatus is controlled by a control rod drive control apparatus. The internal pump is controlled by a recirculation flow rate control apparatus through a inverter controller. In addition, the steam that is generated inside the reactor is supplied into the turbine through the main steam system. The steam exhausted from the turbine is condensed by a condenser. The cooling water generated by the condensation of the steam returned to the reactor via the feed water system and the feed water pump. Also, the amount of steam being supplied into the turbine through the main steam system is controlled by the turbine control apparatus.
Japanese Patent Application Laid-Open No. 6-123797 discloses a feed water control apparatus. The feed water control apparatus comprises: a selected water level setting apparatus in which first reactor water level set value is set lower than second reactor water level set value set by the water level setting apparatus; a switching element for switching second reactor water level set signal from the water level setting apparatus to first reactor water level set signal from the selected water level setting apparatus; and a water level setting changing apparatus for outputting a reactor water level setting change signal which switches the switching element by the inserting operation of the pre-selected control rod. When high reactor water level is not detected by the protection system and the selected control rod is operated, tripping of the turbine and scram of the reactor can be avoided by the control of the feed water control apparatus.
The natural circulation reactor is being developed in an effort to obtain a more economical reactor than the existing boiling water reactor and the advanced boiling water reactor. This natural circulation r reactor does not have devices which circulate the cooling water by pump force such as the internal pump or the jet pump being used in the existing boiling water reactor and the advanced boiling water reactor, nor the accompanying devices and controllers, and thus the structure is simple. Reactor power control of the forced circulation reactor was performed by recirculation flow rate control using the internal pump and the like. However, for that reason, other important control could not be performed in the natural circulation reactor.
In the natural circulation reactor, the reactor power control is mainly performed by the withdrawing operation of the control rod and the inserting operation of the control rod. The reactor power control being used the control rod in the natural circulation reactor occurs some difficulty with power control in the rated reactor power vicinity. It is first difficulty that fine adjustment of the reactor power by the reactor power control using the control rod is difficult. It is second difficulty that there is a large possibility that the maximum linear heat generating ratio or the minimum critical power ratio of the core will be outside the thermal limit range by local heating because comparatively large changes in power distribution occurs due to control rod operation.
Japanese Patent Laid-open No. Sho 63-223591 is described a reactor power control apparatus being devices for supplementing the reactor power control using control rod operation. The reactor power control apparatus adjusts the reactor water level in the coolant descending path so as to be lower than the reactor water level in the core shroud based on the reactor power demand signal. The coolant descending path is formed between the reactor vessel and the core shroud which encloses the core. Note that a coolant descending path returns the cooling water ejected from the core. The reactor power control apparatus controls the reactor power by adjusting the reactor water level so as to be the demanded reactor power and follows the reactor power corresponding load change due to the core flow rate.
In the natural circulation reactor, because the cooling water is circulated by natural circulation force, it is difficult to obtain the core flow rate that is the same as the forced circulation nuclear reactor in which the cooling water is circulated by force using a pump. As a result, in the natural circulation reactor, there is a tendency for the reactor power density to small compared with that of the forced circulation reactor.
As a result, a technique is known in which in order to increase the core flow rate and the reactor power density, a space called a chimney being filled with boiling water is formed above the reactor by extending the core shroud in the vertical direction (see Japanese Patent Laid-open No. 2003-130982 for example). The density difference between the inside and outside the core is increased by the chimney, and thus the core flow rate is increased.
A natural circulation reactor is desired which has excellent operation characteristics compared to the boiling water reactor in addition to economic advantage and maintenance performance. As a result, it is necessary to automate reactivity control in the time of start-up. However as described above, in the reactor power control using the control rod operation, a problem occurs that the reactor power control in the rated reactor power vicinity is difficult, because fine adjustment of the reactor power is difficult, and there is a large possibility that the maximum linear heat generating ratio or the minimum critical power ratio of the core will be outside the thermal limit range by distorted power distribution. For this reason, as described in Japanese Patent Laid-open No. Sho 63-223591, there is a method in which the reactor power control includes the control rod operation and the reactor water level control.
A method for the reactor power control is described in Japanese Patent Laid-open No. Sho 63-223591. In the method, the reactor water level is controlled independently of the control rod operation, either manually or automatically. If the reactor water level is manually or automatically controlled independent of the control rod operation, there is possibility that the measured reactor water level will reach the upper or lower limit of the reactor water level, because the reactor water level decreases by the insertion of the control rod for example, and the water level decreases by the reactor water level control for decreasing the reactor power too. In this manner when the reactor water level decreases based on water level variation caused by a reactor power change due to the control rod operation, and water level change controlled by feed water control, great variation of the reactor water level occurs. Thus, there is a problem in that the reactor water level monitoring will be reached the upper or lower limit of the reactor water level, automatic control stops or the scram of the reactor occurs. In order to prevent automatic control stopping or scramming, it becomes necessary to limit the water level setting range and ensure a margin. However, a problem arises in that the water level setting range that can be utilized in reactor power control becomes small.
In addition, in the case of the natural circulation reactor, because there is no re-circulation pump which circulates the cooling water by force, the core flow rate depends on the reactor water level in the downcomer portion (static head). In a natural circulation reactor, control of the reactor water level is important, because reactivity is applied in the reactor when the core flow rate increases, and because the reactor power increases.
However, in the feed water control apparatus of the conventional forced circulation reactor of Japanese Patent Laid-open No. 2003-130982, the control system is basically one in which the reactor water level is kept constant. Control in which the reactor power control corresponds with the reactor water level was never carried out in the prior art.